Pipe used in pipelines for transporting fluids, such as oil and gas, is usually quite large in diameter, for example, anywhere from one foot to four feet. The gate valve is one form of valve commonly used in pipelines. Gate valves employed heretofor for isolating sections of the pipe of this large diameter have been massive, expensive devices and have not been entirely reliable. The following United States patents describe gate valve structures which can be considered to be representative of the prior art:
U.S. Pat. No. 3,069,129. H. M. Grove, granted Dec. 18, 1962 for "Valve Construction having Fluid Pressure and Spring Bias Seals";
U.S. Pat. No. 3,367,635, R. Fortune, granted Feb. 6, 1968 for "Slide Gate Valves";
U.S. Pat. No. 3,575,377, S. E. Carlton, granted Apr. 20, 1971 for "Closure Seals for Gate Valve and Method";
U.S. Pat. No. 4,062,516, H. M. Grove, granted Dec. 13, 1977 for "Valve Construction"; and
U.S. Pat. No. 4,275,866, D. W. Jamar, granted June 30, 1981 for "Operator for a Gate Valve".
Each of the valves disclosed in these patents suffers from a deficiency which is common to most gate valves. Because the mating and sealing surfaces within the valve are in contact when the gate is moved to and from its closed position, friction galling of these surfaces frequently occurs. This galling destroys the intergrity or uniformity of the valve surfaces and impairs the ability of the valve components to form a perfect seal. Leaky valves are the result, and this simply cannot be tolerated in pipeline installations.
When an isolation valve leaks in its "closed" position it complicates and greatly increases the cost of performing the desired repair or maintenance of the isolated sections of the complex. Repairing a pipeline section or component into which oil or gas is leaking is hazardous and requires great care on the part of the work crew. Often, to shut off the flow through a leaking isolation valve, it is necessary to shut down or isolate other portions of the complex to adequately eliminate or divert the leaking fluids. This can be very costly if it reduces the overall capacity of the pipeline to deliver product.
Another deficiency which is largely inherent in gate valve structures is the limited forces that are available to seat sealing surface against sealing surface in the valve. This is because these sealing surfaces are parallel or nearly parallel to the plane of movement of the valve gate. The patents to Fortune and Carlton, identified above, are addressed to this problem and disclose fluid pressure systems for increasing the seating forces of the valves. Fortune requires a separate source of pressurized fluid and therefore utilizes an expensive installation really not suited for isolated locations, of which there are many in a pipeline complex. Carlton relies on differential forces derived from the pressure of the fluid in the pipeline, which arrangement can generate only limited sealing forces. Consequently, Carlton must rely on fragile rubber O-rings to effect a seal.
There is a need, therefore, for a rugged, reliable, yet relatively inexpensive gate valve capable of effecting an absolute shut off of a pipeline.